At log in, when it says "Connection Failure: Unable to connect to the PVP.net server. Click OK to close the client.". I made a post in the topic related to that error though. I got this Maestro Error only once. Just when I thought I could actually connect and check the game out, BAM! - Buongiorno Maestro lol

EDIT

Apparently Kaspersky Internet Security 7 firewall protection was blocking the LoL client from connecting to Maestro server.
Paused the protection, connected and resumed the protection.
Even though I added the exceptions to Firewall rules in KIS7, port forwarding is also needed.

For others with similar issues: The maestro server is the device that allows pvp.net to communicate with your OS. So generally speaking, these kinds of errors usually have something to do with your OS configuration.

i have the same problem. My virus scanner (kaspersky) updated and i got the same error.
I disabled kaspersky and now, for some reason, it happens occasionally.
It seems to be the first time after booting LoL that i get this messages, restart, and it works. This i am not 100% sure of though and it could be a coincidence, then it just seems random